Rechargeable lithium ion batteries have been commercially available for well over a decade. In spite of the improvements in energy densities and power densities, lithium ion cell technology remains restricted to a narrow temperature range of operation. The electrolyte components ethylene carbonate (EC) and lithium hexafluorophosphate (LiPF6) are responsible for much of the temperature range limitation. As a result, lithium batteries can only deliver the rated capacity and power in a narrow range of −20° C. and +60° C. Below −30° C., lithium cells suffer severe power and energy loss along with the safety risk caused by possible lithium metal deposition; while at temperatures higher than 60° C., the thermal decomposition of LiPF6 causes permanent degradation of the cell and potential safety hazards. These restrictions limit the usage of lithium ion batteries in a variety of harsh environments experienced by electric or hybrid electric vehicles (EV/HEV), military and space missions.
Efforts to lower the low temperature operational limits of Li ion cells have focused on replacing the majority of the high melting EC with high ratios of low melting solvents such as the linear dialkylcarbonates or esters. Exemplary of this effort is U.S. Pat. No. 6,492,064 (Smart et al.). Unfortunately, the cycle life of such modified Li ion cells is compromised at room temperatures. Moreover, the oxidative decomposition of these volatile co-solvents at the charged surface of the cathode accelerates at elevated temperatures, resulting in gas buildup and shortened cell lifetime.
Efforts to raise the high temperature operational limit of Li ion cells include using a thermally stable lithium salt and have been scarce and rarely successful. An example of the use of a thermally stable salt lithium bis(oxalato)borate (LiBOB) is Xu et al., Electrochemical and Solid-State Letters, 5 (1), A26 (2002). While Li ion cells having an electrolyte based on LiBOB and carbonate mixtures such as EC/dimethylene carbonate (DMC) or EC/propylene carbonate (PC)/DMC can stably cycle at temperatures as high as 70° C., such cells do suffer from lower power and diminished low temperature performance.
Another example for the effort to improve the stability of electrolytes at high temperature is shown by Takami et al., who taught an electrolyte including LiBF4 dissolved in a gamma-butyrolactone (GBL) reduces the gas production within a lithium ion cell when the cell is stored at high temperatures. However, such cells also suffer from lower power and diminished low temperature performance. The cycling performance of these lithium ion cells deteriorates rapidly at high temperatures, caused by the presence of labile fluorines in the anion BF4−. (Takami et al., J. Electrochem. Soc., 149, A9 (2002)). The electrolyte combination of GBL with LiPF6 also fails to deliver good performances in lithium ion cells even at room temperature, due to the low stability of the electrolyte on anodic graphite. (Chagnes et al., J. Electrochem. Soc., 150, A1255 (2003)).
Currently, no electrolyte composition is available which can simultaneously support the stable operation of Li ion cells at both high and low temperatures. Thus, there exists a need for an electrolyte composition that can simultaneously support the operation of Li ion cells above 60° C. and below −30° C. without serious degradations in cycle life, energy and power.